I had the same issue that me and my roommate wanted to play together on halo 4 and tried everything that i could find that people suggested none of it worked. So i looked at what the only difference was i was playing on a new slim model xbox and he had an old one. He got a 4gig slim xbox for Christmas and now we can party up just fine. I guess its a hardware compatablity issue not really a network problem. Hope this helps.